The invention is directed to semiconductor lasers with a high side-mode-suppression ratio and narrow-linewidth for telecommunications applications, and more particularly to a Fabry-Perot semiconductor laser that can be post-processed into such a device at the wafer level, i.e. before the wafer is separated into individual dies by cleaving/dicing.
Fabry-Perot (FP) lasers have multiple lasing modes within the envelope of the gain spectrum, with the modes being separated by the Free Spectral Range (FSR), which is determined by the length of the laser cavity between facets and the effective refractive index and the dispersion of the active layer. FP lasers are used today in short-reach (typically Local Area or Wide Area Networks) applications, where their relatively large effective linewidth due to multiple lasing modes around the nominal emission wavelength can be tolerated. However, their large linewidth can cause intersymbol interference due to the cumulative effects with distance of the optical fiber""s chromatic dispersion, thus precluding their use over longer transmission distances, for example, in Metropolitan Area (MA) and Long Haul (LH) networks. A large linewidth can also cause crosstalk problems (or conversely, channel spacing limitations) when the channel spacing in DWDM networks is decreased. Typically, spans in MA and LH networks require the use of single-mode narrow-linewidth laser diodes such as Distributed Feedback (DFB) lasers. DFBs have a grating extending along or close to the active layer, and tend to lase in a single mode and have both a high side-mode-suppression ratio (SMSR) and a narrow linewidth. However, the complex etch and regrowth processes required to write the grating and passivate afterwards before continuing to lay down the upper device layers make DFBs very expensive to manufacture compared to FP lasers.
Attempts have been reported in the past for transforming a FP laser into a quasi-single-mode laser by post-growth processing.
WO 01/18924A1 discloses forming a two-dimensional pattern of etched recesses outside the waveguide region of a semiconductor laser, thereby changing the effective refractive index of the waveguide near the holes. However, the effective index contrast is low and, compared to the technique disclosed herein, the number of features has to be increased by about two orders of magnitude to stabilize single-mode operation.
Another approach for forcing a FP laser into single-mode operation is described by Kozlowski et al. in IEEE Photonics Technology Letters Vol. 8, No. 6, pp. 755-7, 1996. Recesses that extend as far down as the upper edge of the active layer of a FP laser are formed in order to suppress certain modes. However, the design of Kozlowski et al. requires that the recesses be located at an integer fraction of the laser length L, e.g. L/2, L/4 or L/8, from a laser facet. Because the recesses are preferably formed before the wafer is cleaved into laser dies and due to the uncertainty of several microns in the as-cleaved length of any given laser die, this technique can only be performed with accuracy on discrete, mounted laser devices, the lengths of which have been measured. The process is therefore difficult to realize in practical applications and expensive to implement.
Another approach has been proposed by DeChiaro in J. Lightwave Technol., Vol. 9, No. 8, pp. 975-986, August 1991, wherein absorbing regions are introduced at appropriate locations along the optical axis of the device. The absorption regions are also placed at sites that are predetermined integer fractions of the laser length which again requires that the length of the FP laser be accurately known.
It would therefore be desirable to produce a quasi-single-mode laser with a high SMSR from a conventional FP laser without prior definition of the cavity length of the FP laser. It would further be desirable to produce such lasers economically using standard laser processing tools at the wafer level, i.e., before dicing/cleaving the wafer into individual devices.
The systems and methods described herein include systems and methods that control the lasing modes of a Fabry-Perot (FP) laser, and more particularly to a semiconductor FP laser that can be post-processed into such device at the wafer level, i.e. before the wafer is separated into individual laser dies by cleaving/dicing.
According to one aspect of the invention, a laser device includes a laser cavity having an optical waveguide and facets defining first lasing modes and a sub-cavity formed within the laser cavity. The sub-cavity has a predetermined length and is located between the facets, wherein the predetermined sub-cavity length defines second lasing modes having a free spectral range (FSR) that is greater than a FSR of the first lasing modes. The device further includes a plurality of contrast elements having predetermined inter-element separations and predetermined spacings relative to the sub-cavity, wherein the contrast elements interact with the optical waveguide and form an aperiodic grating. The inter-element separations and the spacings produce a filtering function of the aperiodic grating for optical radiation propagating in the waveguide.
The small sub-cavity is designed to expand the mode spacing of the FP laser. In addition, an aperiodic grating filter in the form of contrast elements (typically fewer than 20) is applied along the waveguide, i.e., between the facets and inside and/or outside the sub-cavity, that has a transmission passband at the desired lasing wavelength and substantially a stop-band over a range of adjacent frequencies on both sides of the desired lasing wavelength. The contrast elements can be formed in or adjacent to the waveguide layer of the laser.
Embodiments of the invention can include one or more of the following features. The sub-cavity length can be defined by at least two discontinuities, such as trenches and/or cleaves, extending into the optical waveguide. The waveguide can include an upper cladding layer, wherein the contrast elements comprise regions having an index of refraction that is different from an index of refraction of the upper cladding layer and/or of the waveguide. The filtering function can cause the laser device to emit laser radiation with at least one defined mode. Preferably, the emitted laser radiation can be single-mode. A contact layer can be disposed at least over the length of the sub-cavity, but may also be disposed over the waveguide region located between the facets. The facets can be anti-reflection coated.
The laser device can also be implemented as a surface-emitting laser by forming at least one of the facets as an inclined facet that directs the optical radiation propagating in the waveguide in a direction substantially normal to the waveguide. Alternatively, at least one of the facets can be oriented perpendicular to the waveguide and located opposite to a reflecting facet that directs the optical radiation propagating in the waveguide in a direction substantially normal to the waveguide.
According to another aspect of the invention, a method for producing optical radiation with a desired mode structure includes producing an optical waveguide capable of propagating the optical radiation; forming in the optical waveguide a sub-cavity with a predetermined length, the sub-cavity producing a sub-cavity mode structure that overlaps with the desired mode structure; and forming along the optical waveguide an aperiodic filter having contrast elements, the aperiodic filter filtering the sub-cavity mode structure to generate the desired mode structure. The optical filter is generated by defining a cost function representing a goodness of fit between the desired mode structure and an actual spectral response of the filter; assigning to each contrast element of the aperiodic filter at least one characteristic attribute; generating an initial arrangement of the contrast elements along the optical waveguide; and iteratively modifying at least one characteristic attribute of at least one of the contrast elements and computing for each iteration a cost function. If the computed cost function for an iteration is less than a predetermined cost function value, a corresponding arrangement of the contrast elements is selected as an optimal arrangement of the contrast elements to provide the desired mode structure.
Embodiments can include one or more of the following features. The optimal arrangement of the contrast elements can define a spatial layout of the contrast elements relative to one another and relative to the sub-cavity. Facet locations can be defined along the optical waveguide, the facet locations defining Fabry-Perot modes, wherein the sub-cavity and the optimally arranged contrast elements are located between the facet locations. The distance between the facet locations, on one hand, and the sub-cavity and the optimally arranged contrast elements, on the other hand, need only be known approximately. The characteristic attributes of the contrast element can be a refractive index, a physical dimension, such as a width and/or height, or a spacing between adjacent contrast elements.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments and from the claims.